In conventional or “wet” lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. For example, the ink can be first transferred to an intermediate blanket that in turn is used to transfer the ink to the surface of the material upon which the image is to be reproduced.
Imageable elements useful to prepare lithographic printing plates typically comprise one or more imageable layers applied over the hydrophilic surface of a substrate. The imageable layers include one or more radiation-sensitive components that can be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material. Following imaging, either the imaged regions or the non-imaged regions of the imageable layer are removed by a suitable developer, revealing the underlying hydrophilic surface of the substrate. If the imaged regions are removed, the element is considered as positive-working. Conversely, if the non-imaged regions are removed, the element is considered as negative-working. In each instance, the regions of the imageable layer (that is, the image areas) that remain are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water and aqueous solutions, typically a fountain solution, and repel ink.
Direct digital or thermal imaging has become increasingly important in the printing industry because of their stability to ambient light. The imageable elements for the preparation of lithographic printing plates have been designed to be sensitive to heat or infrared radiation and can be exposed using thermal heads of more usually, infrared laser diodes that image in response to signals from a digital copy of the image in a computer a platesetter. This “computer-to-plate” technology has generally replaced the former technology where masking films were used to image the elements.
These imaging techniques require the use of alkaline developers to remove exposed (positive-working) or non-exposed (negative-working) regions of the imaged layer(s). In some instances of positive-working lithographic printing plate precursors that are designed for IR imaging, compositions comprising infrared radiation-sensitive absorbing compounds (such as IR dyes) inhibits and other dissolution inhibitors make the coating insoluble in alkaline developers and soluble only in the IR-exposed regions.
To enable good solubility in an alkaline developer, one must use a polymeric binder in the imageable layer that is highly soluble the alkaline developer in order to generate a significant difference between the solubility of the non-exposed regions and that of the exposed regions in the alkaline developer. This, however, makes the image more susceptible to developer attack and less resistant to scratches during handling.
Therefore substances that function as image protecting agents, often referred to as dissolution inhibitors, are well known and are commonly used in high pH developers (pH greater than 13) for positive-working lithographic printing plate precursors, many of which are commercially available in the industry. Such printing plate precursors generally include phenolic polymeric binders in the outermost imageable layers. The interaction of the dissolution inhibitors with phenolic groups in the polymeric binders gives good protection of the non-exposed (imaged) regions while exposed (non-image) regions are selectively and readily removed. Polymeric binders having pendant carboxylic acids can also interact with dissolution inhibitors but the interaction are weaker and less useful.
Lower pH developers (pH less than 12) are generally used to process imaged negative-working lithographic printing plate precursors that contain polycondensed diazonium salts or free radical polymerizable polymers in the imageable layer. Such imageable layer can also include polymeric binder having carboxylic groups. There is good differential in solubility between the imaged (exposed) where generally a crosslinking or fusing of particles takes place and non-image (non-exposed) regions, making it unnecessary for the solubility suppressing or dissolution inhibitors in the coating to protect the image.
U.S. Pat. No. 7,582,407 (Savariar-Hauck et al.) and U.S. Pat. No. 7,563,556 (Savariar-Hauck et al.) describe the use of phenolic polymeric binders in positive-working lithographic printing plate precursors that can be processed using low pH developers. To avoid the buildup of sludge in the processing bath, the imageable layers are designed to dissolve completely in the alkaline developer but the resulting image is susceptible to damage from the developer. The dissolution inhibitors mentioned above are not very effective as image protecting agents because the low pH developers generally include alkanolamines, organic solvents, and hydrotropic agents that tend to hinder the interaction of the dissolution inhibitors with phenolic groups of the polymeric binders. These developers can also contain N-alkoxylated or quaternary amines as described in DE 10 310 168 and 19 845 605.
High pH developers can also include various development inhibitors including polyalkyeneoxides and quaternary ammonium salts, as described for example in WO 2003/012550 (Fiebag et al.) Amphoteric dissolution inhibitors are described in WO 2007/023336 (Fiebag et al.). Other high pH developers include poly(vinyl pyrrolidone) as described for example, in EP 323,836A1 (Toyama et al.) and EP 1,449,675A1 (Hauck et al.), or N-heterocyclic compounds as described for example, in DE 3,927,693 (Takeda et al.).
However, generally the activity of high pH developers is difficult to maintain because of neutralization that occurs over time as imaged precursors are developed or from atmospheric carbon dioxide, and thus they must be carefully monitored to enable steady performance. Furthermore, the silicates in such developers can react undesirably with the aluminum support or minerals in the water to form sludge that can clog the plumbing and components of processor equipment.
There remains a need for a way to develop IR-exposed positive-working lithographic printing plate precursors in low pH developers without the disadvantages of high pH developers, but making use of the solubility suppressing agents that are commonly used in high pH developers to provide image protection. It would be particularly desirable to process precursors that include phenolic polymeric binders in the outermost imageable layer that are designed to be soluble in low pH developers.